1. Field of the Invention
The present invention relates to an ink jet recording apparatus for image recording by discharging recording liquid (ink) from a discharge port of a recording head, an ink jet recording head adapted for use in such apparatus, and an ink jet recording method adapted for use in such apparatus.
2. Related Background Art
FIGS. 6, 7A and 7B show an example of conventional ink jet recording apparatus utilizing an ink tank incorporating an ink absorbent member.
The recording means employed in this apparatus is of cartridge type in which a recording head 100 and and ink tank 200 are integrally constructed and are detachably mounted on a carriage. The ink tank 200 of the cartridge houses an ink absorbent member 202, and ink impregnated and contained therein is supplied to the recording head 100.
In FIG. 7B, a recording head chip 100 comprises a discharge unit 102, a supply tank 104, etc. The discharge unit 102 is provided with discharge ports 102A formed on a face opposed to the recording medium, liquid paths extended inwardly therefrom, recording heaters provided as discharge energy generating members respectively in the liquid paths, and a common liquid chamber communicating with the liquid paths. The supply tank 104 serves as a subsidiary tank for receiving the ink from the ink tank 200 and guiding the ink to the common liquid chamber of the discharge unit 102.
The ink absorbent member 202, provided in the ink tank 200 and impregnated with ink, can be composed of a porous material or a fibrous material. A cover member 204 is provided for the ink tank 200.
Referring to FIG. 6, recording head cartridges 14 of the form shown in FIG. 7A are positioned and fixed on a carriage 14 by means of pressing members 41 and are capable of reciprocating along guide shafts 21, in a longitudinal direction, perpendicular to the advancing direction of the recording material. The positioning on the carriage 15 can be achieved, for example, by mutual engagement or mutual pressing of positioning parts formed on the recording head cartridge 14 and the carriage 15. Also, an electrical connection can be made by coupling a connection pad of a printed wiring board (not shown) for the discharge unit 102 with a connector on the carriage 15.
The ink discharged from the discharge ports of the recording head cartridge 14 reaches the recording material 18 which is conveyed by conveying means. The recording face of the recording material 18 is defined by a platen 19 at a small distance from the discharge face of the recording head cartridge 14, and a desired image can be recorded by the relative movement of the recording material 18 and scanning of the recording head cartridge 14. The conveyance of the recording material may be performed by a known conveying mechanism, either by an independent motor or a motor used for carriage driving or for driving of a recovery unit for the recording head.
The recording head cartridge 14 receives discharge signals corresponding to image data from a suitable data source, through a cable 16 and terminals thereof. There may be provided one or plural recording head cartridges 14 (two cartridges 14 are provided in the drawing), according to the characteristics required in the recording, for example mono-color recording, recording with continuous density or full-color recording.
In FIG. 6, there are further shown a carriage motor 17 for moving the carriage 15 along the shafts 21, a wire 22 for transmitting the driving force of the motor 17 to the carriage 15, and a feed motor 20 coupled with the platen roller 19 for conveying the recording material 18.
In the above-explained recording ink cartridge in which the ink tank and the recording head are mutually connected, a negative pressure acts on the ink in the recording head, due to the capillary action of the ink absorbent member of the ink tank, thus applying a force to suck the ink from the recording head side toward the absorbent member in the ink tank. Thus, the balance between the negative pressure and the sucking pressure resulting from the capillary action of the liquid paths of the recording head side meniscuses the ink surfaces in the discharge ports.
In such recording means in which the meniscus of ink is maintained by the balance of the capillary action of the ink tank side provided with the ink absorbent member and the capillary action of the ink paths of the recording head side, the liquid head pressure at the discharge ports varies according to the amount of ink in the ink tank, due to the change in the negative pressure applied from the ink tank side to the ink paths communicating with the discharge ports.
The remaining amount of ink in the ink tank and the head pressure of ink at the discharge port are generally correlated as indicated in FIG. 8. More specifically, the head pressure at the discharge port is lowered with a decrease in the remaining amount of ink in the ink tank.
In FIG. 8, the curves a and b indicate "behaviors" of the head pressure with absorbent materials of different absorbent abilities, whereby the curve b respresents a larger absorbent ability. It will be understood that the change in the head pressure becomes larger as the absorbent ability increases. Also FIG. 9 shows the relationship between the head pressure and the amount of ink discharge (quantity per ink droplet) from the discharge port of the recording head. The ink discharge amount gradually decreases with the decrease of the head pressure and may become zero at a certain head pressure. Such a phenomenon is caused by the decrease of head pressure at the discharge port, resulting from an increase of the negative pressure in the ink tank, caused by the decrease of the remaining amount of ink in the ink tank. This is because an increase in the negative pressure of the ink tank side destroys the balance of pressure between the ink tank side and the discharge port side, thus increasing the sucking force on the ink toward the ink tank side. Such increased sucking force on the ink toward the ink tank side reduces the ink discharge amount, if the ink discharge power given to the recording head is maintained constant. With a further decrease in the ink head pressure, the capillary absorbing force of the ink paths becomes comparable to the negative pressure, so that the flow into the ink paths after ink discharge is retarded, thus prolonging the ink refill time, required for replenishing the discharged ink. Since sufficiently rapid refilling cannot therefore be achieved with a constant driving frequency, the ink discharge amount decreases rapidly, and ink discharge may eventually become impossible even though the ink remains in the ink tank.
FIG. 10 shows the relationship between the head pressure and the refilling frequency.
As explained in the foregoing, in an ink jet recording apparatus utilizing a cartridge in which an ink tank incorporating an ink absorbent member and a recording head are integrally connected, there may be encountered a variation in the ink discharge amount depending on the remaining amount of ink in the ink tank. Such variation in the ink discharge amount is directly reflected in the density of the printed image, and is often regarded as a technical problem to be solved in an image output apparatus used for halftone images.
Also, as will be understood from the curves in FIG. 8, the use of an ink absorbent material with a larger absorbent ability for the purpose of increasing the ink amount contained in the ink tank, thereby reducing the operating cost at the device, will result in a larger variation in the ink discharge amount, thus influencing to an even greater extent the density or contrast of the printed image.
Also, before the ink in the ink tank can be efficiently used up, the negative pressure of the ink tank side may increase to disable the ink discharge, thus reducing the efficiency of ink utilization.